Policy and enforcement approaches are often advocated as best practices to reduce underage drinking. Research indicates that there is considerable variability in alcohol policy and enforcement at the state and local level, but few studies have examined whether variations in local policies and enforcement activities are related to differences in alcohol use and related problems among young people. Apparently no studies have investigated whether local alcohol policies and/or enforcement activities influence initiation to drinking or drinking trajectories. Very little is known about the mediating mechanisms (e.g., sources of alcohol, perceived availability, perceived enforcement, perceived norms) through which environmental interventions may affect youth alcohol use. Additionally, no studies have investigated the extent to which local alcohol policies may interact with background and personal characteristics of young people (e.g., age, gender, ethnicity) and levels of enforcement. Such moderating effects may be important for understanding how local policies influence youth access to alcohol and underage drinking. To address these questions, the proposed study will use annual assessments of local regulatory policies, practices, and enforcement activities based on archival and key informant survey data. Adolescents' drinking beliefs, behaviors, and problems will be measured using annual telephone surveys in the same cities. The specific aims of the study are to (1) describe and quantify local alcohol regulatory policies, practices, and enforcement activities in a sample of 50 California cities and document changes in these policies over time; (2) determine whether local regulatory policies, practices, and enforcement activities are associated with differences in underage drinking and drinking problems and changes in drinking behaviors and problems over time; (3) investigate whether differences in local alcohol policies and enforcement are related to potentially important intermediate factors (e.g., sources of alcohol, perceived availability, perceived enforcement, alcohol expectancies, perceived norms) and to changes in these factors over time; (4) investigate the extent to which relationships of policies with drinking behaviors, and changes in drinking behaviors and problems, are mediated through these intermediate factors; and (5) investigate whether age, gender, and ethnic differences moderate the relationships between local regulatory policies, practices and enforcement activities and drinking beliefs and behaviors, and changes in these beliefs and behaviors, over time. A secondary aim of the study is to explore community characteristics (e.g., availability of resources, disorganization, ethnic diversity, presence of coalitions) that may be related to the adoption of more comprehensive policies. This study will help to identify effective policy practices at the local level for reducing underage drinking and alcohol-related problems. The ultimate goal of this research to identify effective policy and enforcement practices that can be used at the local level for preventing and reducing underage drinking and drinking problems.